Electroactive polymer transducers and actuators

ABSTRACT

The present invention relates to electroactive polymers that are pre-strained to improve conversion from electrical to mechanical energy. When a voltage is applied to electrodes contacting a pre-strained polymer, the polymer deflects. This deflection may be used to do mechanical work. The pre-strain improves the mechanical response of an electroactive polymer. The present invention also relates to actuators including an electroactive polymer and mechanical coupling to convert deflection of the polymer into mechanical work. The present invention further relates to compliant electrodes that conform to the shape of a polymer. The present invention provides methods for fabricating electromechanical devices including one or more electroactive polymers.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to and is a divisionalapplication of U.S. application Ser. No. 09/620,025, filed Jul. 20, 2000and entitled, “ELECTROACTIVE POLYMER TRANSDUCERS AND ACTUATORS, which isincorporated herein for all purposes, which claimed priority under 35U.S.C. §119(e) from co-pending U.S. Provisional Patent Application No.60/144,556 filed Jul. 20, 1999, naming R. E. Pelrine et al. asinventors, and titled “High-speed Electrically Actuated Polymers andMethod of Use”, which is incorporated by reference herein for allpurposes

[0002] and which claimed priority under 35 U.S.C. §119(e) fromco-pending U.S. Provisional Patent Application No. 60/153,329 filed Sep.10, 1999, naming R. E. Pelrine et al. as inventors, and titled“Electrostrictive Polymers As Microactuators”, which is incorporated byreference herein for all purposes

[0003] and which claimed priority under 35 U.S.C. §119(e) fromco-pending U.S. Provisional Patent Application No. 60/161,325 filed Oct.25, 1999, naming R. E. Pelrine et al. as inventors, and titled“Artificial Muscle Microactuators”, which is incorporated by referenceherein for all purposes

[0004] and which claimed priority under 35 U.S.C. §119(e) fromco-pending U.S. Provisional Patent Application No. 60/181,404 filed Feb.9, 2000, naming R. D. Kornbluh et al. as inventors, and titled “FieldActuated Elastomeric Polymers”, which is incorporated by referenceherein for all purposes;

[0005] and which claimed priority under 35 U.S.C. §119(e) fromco-pending U.S. Provisional Patent Application No. 60/187,809 filed Mar.8, 2000, naming R. E. Pelrine et al. as inventors, and titled “PolymerActuators and Materials”, which is incorporated by reference herein forall purposes;

[0006] and which claimed priority under 35 U.S.C. §119(e) fromco-pending U.S. Provisional Patent Application No. 60/192,237 filed Mar.27, 2000, naming R. D. Kornbluh et al. as inventors, and titled “PolymerActuators and Materials II”, which is incorporated by reference hereinfor all purposes.

[0007] This application cross references co-pending U.S. patentapplication entitled “Elastomeric Dielectric Polymer Film SonicActuator” naming R. E. Pelrine et al. as inventors, filed on Jul. 19,1999 (U.S. application Ser. No. 09/356,801), which claims priority fromPCT/US98/02311 filed Feb. 2, 1998, which claims priority from U.S.Provisional Application No. 60/037,400 filed Feb. 7, 1997, all of whichare incorporated by reference herein.

[0008] This invention is related to U.S. patent application Ser. No.09/619,846, filed Jul. 20, 2000, naming R. Pelrine et al. as inventors.That application is incorporated herein by reference in its entirety andfor all purposes.

[0009] This invention is also related to U.S. patent application Ser.No. 09/619,848, filed Jul. 20, 2000, naming R. Pelrine et al. asinventors. That application is incorporated herein by reference in itsentirety and for all purposes.

[0010] This invention is also related to U.S. patent application Ser.No. 09/619,843, filed Jul. 20, 2000, naming R. Pelrine et al. asinventors. That application is incorporated herein by reference in itsentirety and for all purposes.

[0011] This invention is also related to U.S. patent application Ser.No. 09/619,845, filed Jul. 20, 2000, naming R. Pelrine et al. asinventors. That application is incorporated herein by reference in itsentirety and for all purposes.

[0012] This invention is also related to U.S. patent application Ser.No. 09/619,847, filed Jul. 20, 2000, naming Q. Pei et al. as inventors.That application is incorporated herein by reference in its entirety andfor all purposes.

BACKGROUND OF THE INVENTION

[0013] The present invention relates generally to electroactive polymersthat convert from electrical energy to mechanical energy. Moreparticularly, the present invention relates to pre-strained polymers andtheir use in actuators and various applications. The present inventionalso relates to compliant electrodes used to electrically communicatewith electroactive polymers and methods of fabricating pre-strainedpolymers.

[0014] In many applications, it is desirable to convert from electricalenergy to mechanical energy. Exemplary applications requiringtranslation from electrical to mechanical energy include robotics,pumps, speakers, general automation, disk drives and prosthetic devices.These applications include one or more actuators that convert electricalenergy into mechanical work—on a macroscopic or microscopic level.Common electric actuator technologies, such as electromagnetic motorsand solenoids, are not suitable for many of these applications, e.g.,when the required device size is small (e.g., micro or mesoscalemachines). These technologies are also not ideal when a large number ofdevices must be integrated into a single structure or under variousperformance conditions such as when high power density output isrequired at relatively low frequencies.

[0015] Several ‘smart materials’ have been used to convert betweenelectrical and mechanical energy with limited success. These smartmaterials include piezoelectric ceramics, shape memory alloys andmagnetostrictive materials. However, each smart material has a number oflimitations that prevent its broad usage. Certain piezoelectricceramics, such as lead zirconium titanate (PZT), have been used toconvert electrical to mechanical energy. While having suitableefficiency for a few applications, these piezoelectric ceramics aretypically limited to a strain below about 1.6 percent and are often notsuitable for applications requiring greater strains than this. Inaddition, the high density of these materials often eliminates them fromapplications requiring low weight. Irradiated polyvinylidene difluoride(PVDF) is an electroactive polymer reported to have a strain of up to 4percent when converting from electrical to mechanical energy. Similar tothe piezoelectric ceramics, the PVDF is often not suitable forapplications requiring strains greater than 4 percent. Shape memoryalloys, such as nitinol, are capable of large strains and force outputs.These shape memory alloys have been limited from broad use byunacceptable energy efficiency, poor response time and prohibitive cost.

[0016] In addition to the performance limitations of piezoelectricceramics and irradiated PVDF, their fabrication often presents a barrierto acceptability. Single crystal piezoelectric ceramics must be grown athigh temperatures coupled with a very slow cooling down process.Irradiated PVDF must be exposed to an electron beam for processing. Boththese processes are expensive and complex and may limit acceptability ofthese materials.

[0017] In view of the foregoing, alternative devices that convert fromelectrical to mechanical energy would be desirable.

SUMMARY OF THE INVENTION

[0018] In one aspect, the present invention relates to polymers that arepre-strained to improve conversion between electrical and mechanicalenergy. When a voltage is applied to electrodes contacting apre-strained polymer, the polymer deflects. This deflection may be usedto do mechanical work. The pre-strain improves the mechanical responseof an electroactive polymer relative to a non-strained polymer. Thepre-strain may vary in different directions of a polymer to varyresponse of the polymer to the applied voltage.

[0019] In another aspect, the present invention relates to actuatorscomprising an electroactive polymer and mechanical coupling to convertdeflection of the polymer into mechanical output. Several actuatorsinclude mechanical coupling that improves the performance of anelectroactive polymer.

[0020] In yet another aspect, the present invention relates to compliantelectrodes that conform to the changing shape of a polymer. Many of theelectrodes are capable of maintaining electrical communication at thehigh deflections encountered with pre-strained polymers of the presentinvention. In some embodiments, electrode compliance may vary withdirection.

[0021] In another aspect, the present invention provides methods forfabricating electromechanical devices including one or moreelectroactive polymers. Pre-strain may be achieved by a number oftechniques such as mechanically stretching a polymer and fixing thepolymer to one or more solid members while it is stretched. Polymers ofthe present invention may be made by casting, dipping, spin coating,spraying or other known processes for fabrication of thin polymerlayers. In one embodiment, a pre-strained polymer comprises acommercially available polymer that is pre-strained during fabrication.

[0022] In another aspect, the present invention relates to a transducerfor translating from electrical energy to mechanical energy. Thetransducer includes at least two electrodes and a polymer arranged in amanner which causes a portion of the polymer to deflect in response to achange in electric field. The polymer is elastically pre-strained.

[0023] In another aspect, the present invention relates to a transducerfor converting from electrical energy to mechanical energy. Thetransducer comprises at least two electrodes and a polymer arranged in amanner which causes a portion of the polymer to deflect in response to achange in electric field provided by the at least two electrodes. Theportion of the polymer deflects with a maximum linear strain betweenabout 50 percent and about 215 percent in response to the change inelectric field.

[0024] In yet another aspect, the present invention relates to anactuator for converting electrical energy into displacement in a firstdirection. The actuator comprises at least one transducer. Eachtransducer comprises at least two electrodes and a polymer arranged in amanner which causes a portion of the polymer to deflect in response to achange in electric field. The actuator also comprises a flexible framecoupled to the at least one transducer, the frame providing mechanicalassistance to improve displacement in the first direction.

[0025] In another aspect, the present invention relates to an actuatorfor converting electrical energy into mechanical energy. The actuatorcomprises a flexible member having fixed end and a free end, theflexible member comprising at least two electrodes and a pre-strainedpolymer arranged in a manner which causes a portion of the polymer todeflect in response to a change in electric field provided by the atleast two electrodes.

[0026] In another aspect, the present invention relates to an actuatorfor converting electrical energy into displacement in a first direction.The actuator comprises at least one transducer. Each transducercomprises at least two electrodes and a polymer arranged in a mannerwhich causes a portion of the polymer to deflect in response to a changein electric field. The actuator also comprises at least one stiff membercoupled to the at least one transducer, the at least one stiff membersubstantially preventing displacement in a second direction.

[0027] In yet another aspect, the present invention relates to adiaphragm actuator for converting electrical energy into mechanicalenergy. The actuator comprises at least one transducer. Each transducercomprises at least two electrodes and a pre-strained polymer arranged ina manner which causes a first portion of the polymer to deflect inresponse to a change in electric field. The actuator also comprises aframe attached to a second portion of the polymer, the frame includingat least one circular hole, wherein the first portion deflects out ofthe plane of the at least one circular hole in response to the change inelectric field.

[0028] In another aspect, the present invention relates to an actuatorfor converting electrical energy into mechanical energy, the actuatorcomprising a body having at least one degree of freedom between a firstbody portion and a second body portion, the body including at least onetransducer attached to the first portion and the second portion, eachtransducer comprising at least two electrodes and a pre-strained polymerarranged in a manner which causes a portion of the polymer to deflect inresponse to a change in electric field; the actuator also comprising afirst clamp attached to the first body portion and a second clampattached to the second body portion.

[0029] In yet another aspect, the present invention relates to anactuator for converting electrical energy to mechanical energy, theactuator comprising a transducer, the transducer comprising a polymerarranged in a manner which causes a first portion of the polymer todeflect in response to a change in electric field, a first electrodepair configured to actuate a second portion of the polymer and a secondelectrode pair configured to actuate a third portion of the polymer, theactuator also comprising an output member coupled to a first portion ofthe polymer.

[0030] In another aspect, the present invention relates to an electrodefor use with an electroactive polymer. The electrode comprises acompliant portion in contact with the electroactive polymer, wherein thecompliant portion is capable of deflection with a strain of at leastabout 50 percent.

[0031] In yet another aspect, the present invention relates to anelectrode for use with an electroactive polymer. The electrodecomprising a compliant portion in contact with the electroactivepolymer, wherein the electrode comprises an opacity which varies withdeflection.

[0032] In yet another aspect, the present invention relates to anelectrode for use with an electroactive polymer. The electrodecomprising a compliant portion in contact with the electroactivepolymer, wherein the compliant portion comprises a textured surface.

[0033] In another aspect, the present invention relates to a method offabricating a transducer including a pre-strained polymer. The methodcomprises pre-straining an electroactive polymer to form thepre-strained polymer. The method also comprises fixing a portion of thepre-strained polymer to a solid member. The method additionallycomprises forming one or more electrodes on the pre-strained polymer.

[0034] In still another aspect, the present invention relates to amethod of fabricating a transducer comprising multiple pre-strainedpolymers. The method comprises pre-straining a first polymer to form afirst pre-strained polymer. The method also comprises forming one ormore electrodes on the first pre-strained polymer. The method furthercomprises pre-straining a second polymer to form a second pre-strainedpolymer. The method additionally comprises forming one or moreelectrodes on the second pre-strained polymer. The method furthercomprises coupling the first pre-strained polymer to the secondpre-strained polymer.

[0035] These and other features and advantages of the present inventionwill be described in the following description of the invention andassociated figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIGS. 1A and 1B illustrate a top perspective view of a transducerbefore and after application of a voltage in accordance with oneembodiment of the present invention.

[0037]FIG. 1C illustrates a textured surface for an electroactivepolymer having a wavelike profile.

[0038]FIG. 1D illustrates an electroactive polymer including a texturedsurface having random texturing.

[0039]FIG. 1E illustrates a cross-sectional side view of a diaphragmactuator including an electroactive polymer before application of avoltage in accordance with one embodiment of the present invention.

[0040]FIG. 1F illustrates a cross-sectional view of the electroactivepolymer diaphragm of FIG. 1E after application of a voltage inaccordance with one embodiment of the present invention.

[0041]FIGS. 2A and 2B illustrate a bow actuator before and afteractuation in accordance with a specific embodiment of the presentinvention.

[0042]FIG. 2C illustrates a bow actuator including additional componentsto improve deflection in accordance with a specific embodiment of thepresent invention.

[0043]FIG. 2D and 2E illustrate a linear motion actuator before andafter actuation in accordance with a specific embodiment of the presentinvention.

[0044]FIG. 2F illustrates a cross-sectional side view of an actuatorincluding multiple polymer layers in accordance with one embodiment ofthe present invention.

[0045]FIG. 2G illustrates a stacked multilayer actuator as an example ofartificial muscle in accordance with one embodiment of the presentinvention.

[0046]FIG. 2H illustrates a linear actuator comprising an electroactivepolymer diaphragm in accordance with another embodiment of the presentinvention.

[0047]FIG. 2I illustrates an inchworm-type actuator including a rolledelectroactive polymer in accordance with one embodiment of the presentinvention.

[0048]FIG. 2J illustrates a stretched film actuator for providingdeflection in one direction in accordance with another embodiment of thepresent invention.

[0049]FIG. 2K illustrates a bending beam actuator in accordance withanother embodiment of the present invention.

[0050]FIG. 2L illustrates the bending beam actuator of FIG. 2K with a 90degree bending angle.

[0051]FIG. 2M illustrates a bending beam actuator including two polymerlayers in accordance with another embodiment of the present invention.

[0052]FIG. 3 illustrates a structured electrode that providesone-directional compliance according to a specific embodiment of thepresent invention.

[0053]FIG. 4 illustrates a pre-strained polymer comprising a structuredelectrode that is not directionally compliant according to a specificembodiment of the present invention.

[0054]FIG. 5 illustrates textured electrodes in accordance with oneembodiment of the present invention.

[0055]FIG. 6 illustrates a two-stage cascaded pumping system includingtwo diaphragm actuator pumps in accordance with a specific embodiment ofthe present invention.

[0056]FIG. 7A illustrates a process flow for fabricating anelectromechanical device having at least one pre-strained polymer inaccordance with one embodiment of the present invention.

[0057] FIGS. 7B-F illustrate a process for fabricating anelectromechanical device having multiple polymer layers in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] The present invention will now be described in detail withreference to a few preferred embodiments thereof as illustrated in theaccompanying drawings. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one skilled inthe art, that the present invention may be practiced without some or allof these specific details. In other instances, well known process stepsand/or structures have not been described in detail in order to notunnecessarily obscure the present invention.

[0059] 1. Overview

[0060] Electroactive polymers deflect when actuated by electricalenergy. In one embodiment, an electroactive polymer refers to a polymerthat acts as an insulating dielectric between two electrodes and maydeflect upon application of a voltage difference between the twoelectrodes. In one aspect, the present invention relates to polymersthat are pre-strained to improve conversion between electrical andmechanical energy. The pre-strain improves the mechanical response of anelectroactive polymer relative to a non-strained electroactive polymer.The improved mechanical response enables greater mechanical work for anelectroactive polymer, e.g., larger deflections and actuation pressures.For example, linear strains of at least about 200 percent and areastrains of at least about 300 percent are possible with pre-strainedpolymers of the present invention. The pre-strain may vary in differentdirections of a polymer. Combining directional variability of thepre-strain, different ways to constrain a polymer, scalability ofelectroactive polymers to both micro and macro levels, and differentpolymer orientations (e.g., rolling or stacking individual polymerlayers) permits a broad range of actuators that convert electricalenergy into mechanical work. These actuators find use in a wide range ofapplications.

[0061] As the electroactive polymers of the present invention maydeflect at linear strains of at least about 200 percent, electrodesattached to the polymers should also deflect without compromisingmechanical or electrical performance. Correspondingly, in anotheraspect, the present invention relates to compliant electrodes thatconform to the shape of an electroactive polymer they are attached to.The electrodes are capable of maintaining electrical communication evenat the high deflections encountered with pre-strained polymers of thepresent invention. By way of example, strains of at least about 50percent are common with electrodes of the present invention. In someembodiments, compliance provided by the electrodes may vary withdirection.

[0062] As the pre-strained polymers are suitable for use in both themicro and macro scales, in a wide variety of actuators and in a broadrange of applications, fabrication processes used with the presentinvention vary greatly. In another aspect, the present inventionprovides methods for fabricating electromechanical devices including oneor more pre-strained polymers. Pre-strain may be achieved by a number oftechniques such as mechanically stretching an electroactive polymer andfixing the polymer to one or more solid members while it is stretched.

[0063] 2. General Structure of Devices

[0064]FIGS. 1A and 1B illustrate a top perspective view of a transducer100 in accordance with one embodiment of the present invention. Thetransducer 100 includes a polymer 102 for translating between electricalenergy and mechanical energy. Top and bottom electrodes 104 and 106 areattached to the electroactive polymer 102 on its top and bottom surfacesrespectively to provide a voltage difference across a portion of thepolymer 102. The polymer 102 deflects with a change in electric fieldprovided by the top and bottom electrodes 104 and 106. Deflection of thetransducer 100 in response to a change in electric field provided by theelectrodes 104 and 106 is referred to as actuation. As the polymer 102changes in size, the deflection may be used to produce mechanical work.

[0065]FIG. 1B illustrates a top perspective view of the transducer 100including deflection in response to a change in electric field.Generally speaking, deflection refers to any displacement, expansion,contraction, torsion, linear or area strain, or any other deformation ofa portion of the polymer 102. The change in electric field correspondingto the voltage difference produced by the electrodes 104 and 106produces mechanical pressure within the pre-strained polymer 102. Inthis case, the unlike electrical charges produced by the electrodes 104and 106 are attracted to each other and provide a compressive forcebetween the electrodes 104 and 106 and an expansion force on the polymer102 in planar directions 108 and 110, causing the polymer 102 tocompress between the electrodes 104 and 106 and stretch in the planardirections 108 and 110.

[0066] In some cases, the electrodes 104 and 106 cover a limited portionof the polymer 102 relative to the total area of the polymer. This maydone to prevent electrical breakdown around the edge of polymer 102 orachieve customized deflections in certain portions of the polymer. Asthe term is used herein, an active region is defined as a portion of thepolymer material 102 having sufficient electrostatic force to enabledeflection of the portion. As will be described below, a polymer of thepresent invention may have multiple active regions. Polymer 102 materialoutside an active area may act as an external spring force on the activearea during deflection. More specifically, material outside the activearea may resist active area deflection by its contraction or expansion.Removal of the voltage difference and the induced charge causes thereverse effects.

[0067] The electrodes 104 and 106 are compliant and change shape withthe polymer 102. The configuration of the polymer 102 and the electrodes104 and 106 provides for increasing polymer 102 response withdeflection. More specifically, as the transducer 100 deflects,compression of the polymer 102 brings the opposite charges of theelectrodes 104 and 106 closer and stretching of the polymer 102separates similar charges in each electrode. In one embodiment, one ofthe electrodes 104 and 106 is ground.

[0068] Generally speaking, the transducer 100 continues to deflect untilmechanical forces balance the electrostatic forces driving thedeflection. The mechanical forces include elastic restoring forces ofthe polymer 102 material, the compliance of the electrodes 104 and 106,and any external resistance provided by a device and/or load coupled tothe transducer 100. The resultant deflection of the transducer 100 as aresult of the applied voltage may also depend on a number of otherfactors such as the polymer 102 dielectric constant and the polymer 102size.

[0069] Electroactive polymers in accordance with the present inventionare capable of deflection in any direction. After application of thevoltage between the electrodes 104 and 106, the electroactive polymer102 increases in size in both planar directions 108 and 110. In somecases, the electroactive polymer 102 is incompressible, e.g. has asubstantially constant volume under stress. In this case, the polymer102 decreases in thickness as a result of the expansion in the planardirections 108 and 110. It should be noted that the present invention isnot limited to incompressible polymers and deflection of the polymer 102may not conform to such a simple relationship.

[0070] The electroactive polymer 102 is pre-strained. The pre-strainimproves conversion between electrical and mechanical energy. In oneembodiment, pre-strain improves the dielectric strength of the polymer.For the transducer 100, the pre-strain allows the electroactive polymer102 to deflect more and provide greater mechanical work. Pre-strain of apolymer may be described in one or more directions as the change indimension in that direction after pre-straining relative to thedimension in that direction before pre-straining. The pre-strain maycomprise elastic deformation of the polymer 102 and be formed, forexample, by stretching the polymer in tension and fixing one or more ofthe edges while stretched. In one embodiment, the pre-strain is elastic.After actuation, an elastically pre-strained polymer could, inprinciple, be unfixed and return to its original state. The pre-strainmay be imposed at the boundaries using a rigid frame or may beimplemented locally for a portion of the polymer.

[0071] In one embodiment, pre-strain is applied uniformly over a portionof the polymer 102 to produce an isotropic pre-strained polymer. By wayof example, an acrylic elastomeric polymer may be stretched by 200-400percent in both planar directions. In another embodiment, pre-strain isapplied unequally in different directions for a portion of the polymer102 to produce an anisotropic pre-strained polymer. In this case, thepolymer 102 may deflect greater in one direction than another whenactuated. While not wishing to be bound by theory, it is believed thatpre-straining a polymer in one direction may increase the stiffness ofthe polymer in the pre-strain direction. Correspondingly, the polymer isrelatively stiffer in the high pre-strain direction and more compliantin the low pre-strain direction and, upon actuation, the majority ofdeflection occurs in the low pre-strain direction. In one embodiment,the transducer 100 enhances deflection in the direction 108 byexploiting large pre-strain in the perpendicular direction 110. By wayof example, an acrylic elastomeric polymer used as the transducer 100may be stretched by 100 percent in the direction 108 and by 500 percentin the perpendicular direction 110. Construction of the transducer 100and geometric edge constraints may also affect directional deflection aswill be described below with respect to actuators.

[0072] The quantity of pre-strain for a polymer may be based on theelectroactive polymer and the desired performance of the polymer in anactuator or application. For some polymers of the present invention,pre-strain in one or more directions may range from −100 percent to 600percent. By way of example, for a VHB acrylic elastomer having isotropicpre-strain, pre-strains of at least about 100 percent, and preferablybetween about 200-400 percent, may be used in each direction. In oneembodiment, the polymer is pre-strained by a factor in the range ofabout 1.5 times to 50 times the original area. For an anisotropicacrylic pre-strained to enhance actuation in a compliant direction,pre-strains between about 400-500 percent may be used in the stiffeneddirection and pre-strains between about 20-200 percent may be used inthe compliant direction. In some cases, pre-strain may be added in onedirection such that a negative pre-strain occurs in another direction,e.g. 600 percent in one direction coupled with −100 percent in anorthogonal direction. In these cases, the net change in area due to thepre-strain is typically positive.

[0073] Pre-strain may affect other properties of the polymer 102. Largepre-strains may change the elastic properties of the polymer and bringit into a stiffer regime with lower viscoelastic losses. For somepolymers, pre-strain increases the electrical breakdown strength of thepolymer 102, which allows for higher electric fields to be used withinthe polymer—permitting higher actuation pressures and higherdeflections.

[0074] Linear strain and area strain may be used to describe thedeflection of a pre-strained polymer. As the term is used herein, linearstrain of a pre-strained polymer refers to the deflection per unitlength along a line of deflection relative to the unactuated state.Maximum linear strains (tensile or compressive) of at least about 50percent are common for pre-strained polymers of the present invention.Of course, a polymer may deflect with a strain less than the maximum,and the strain may be adjusted by adjusting the applied voltage. Forsome pre-strained polymers, maximum linear strains of at least about 100percent are common. For polymers such as VHB 4910 as produced by 3MCorporation of St. Paul, Minn., maximum linear strains in the range of40 to 215 percent are common. Area strain of an electroactive polymerrefers to the change in planar area, e.g. the change in the planedefined by directions 108 and 110 in FIGS. 1A and 1B, per unit area ofthe polymer upon actuation relative to the unactuated state. Maximumarea strains of at least about 100 percent are possible for pre-strainedpolymers of the present invention. For some pre-strained polymers,maximum area strains in the range of 70 to 330 percent are common.

[0075] Generally, after the polymer is pre-strained, it may be fixed toone or more objects. Each object may be suitably stiff to maintain thelevel of pre-strain desired in the polymer. The polymer may be fixed tothe one or more objects according to any conventional method known inthe art such as a chemical adhesive, an adhesive layer or material,mechanical attachment, etc.

[0076] Transducers and pre-strained polymers of the present inventionare not limited to any particular geometry or linear deflection. Forexample, the polymer and electrodes may be formed into any geometry orshape including tubes and rolls, stretched polymers attached betweenmultiple rigid structures, stretched polymers attached across a frame ofany geometry—including curved or complex geometries, across a framehaving one or more joints, etc. Deflection of a transducer according tothe present invention includes linear expansion and compression in oneor more directions, bending, axial deflection when the polymer isrolled, deflection out of a hole provided in a substrate, etc.Deflection of a transducer may be affected by how the polymer isconstrained by a frame or rigid structures attached to the polymer. Inone embodiment, a flexible material that is stiffer in elongation thanthe polymer is attached to one side of a transducer induces bending whenthe polymer is actuated. In another embodiment, a transducer thatdeflects radially out of the plane is referred to as a diaphragm. Adiaphragm actuator will be described in more detail with respect toFIGS. 1E and 1F.

[0077] Transducers (including methods of using them and methods offabricating them) in accordance with the present invention are describedin reports available from the New Energy and Industrial TechnologyDevelopment Organization (NEDO) offices under the reference title“Annual Research Progress Report for R&D of Micromachine Technology (R&Dof High Functional Maintenance System for Power Plant Facilities)” for1999, the “Annual Research Progress Report for R&D of MicromachineTechnology (R&D of High Functional Maintenance System for Power PlantFacilities)” for 1998, the “Annual Research Progress Report for R&D ofMicromachine Technology (R&D of High Functional Maintenance System forPower Plant Facilities)” for 1997, or the “Annual Research ProgressReport for R&D of Micromachine Technology (R&D of High FunctionalMaintenance System for Power Plant Facilities)” for 1996, all of whichare incorporated herein for all purposes. NEDO has several offices inJapan in addition to other offices in the United Sates, Australia,France, Thailand and China.

[0078] Electroactive polymers in accordance with one embodiment of thepresent invention may include a textured surface. FIG. 1C illustrates atextured surface 150 for an electroactive polymer 152 having a wavelikeprofile. The textured surface 150 allows the polymer 152 to deflectusing bending of surface waves 154. Bending of the surface waves 154provides directional compliance in a direction 155 with less resistancethan bulk stretching for a stiff electrode attached to the polymer 152in the direction 155. The textured surface 150 may be characterized bytroughs and crests, for example, about 0.1 micrometer to 40 micrometerswide and about 0.1 micrometers to 20 micrometers deep. In this case, thewave width and depth is substantially less than the thickness of thepolymer. In a specific embodiment, the troughs and crests areapproximately 10 micrometers wide and six micrometers deep on a polymerlayer with a thickness of 200 micrometers.

[0079] In one embodiment, a thin layer of stiff material 156, such as anelectrode, is attached to the polymer 152 to provide the wavelikeprofile. During fabrication, the electroactive polymer is stretched morethan it can stretch when actuated, and the thin layer of stiff material156 is attached to the stretched polymer 152 surface. Subsequently, thepolymer 152 is relaxed and the structure buckles to provide the texturedsurface.

[0080] In general, a textured surface may comprise any non-uniform ornon-smooth surface topography that allows a polymer to deflect usingdeformation in the polymer surface. By way of example, FIG. 1Dillustrates an electroactive polymer 160 including a roughened surface161 having random texturing. The roughened surface 160 allows for planardeflection that is not directionally compliant. Advantageously,deformation in surface topography may allow deflection of a stiffelectrode with less resistance than bulk stretching or compression. Itshould be noted that deflection of a pre-strained polymer having atextured surface may comprise a combination of surface deformation andbulk stretching of the polymer.

[0081] Textured or non-uniform surfaces for the polymer may also allowthe use of a barrier layer and/or electrodes that rely on deformation ofthe textured surfaces. The electrodes may include metals that bendaccording to the geometry of the polymer surface. The barrier layer maybe used to block charge in the event of local electrical breakdown inthe pre-strained polymer material.

[0082] Materials suitable for use as a pre-strained polymer with thepresent invention may include any substantially insulating polymer orrubber that deforms in response to an electrostatic force or whosedeformation results in a change in electric field. One suitable materialis NuSil CF19-2186 as provided by NuSil Technology of Carpenteria,Calif. Other exemplary materials suitable for use as a pre-strainedpolymer include, any dielectric elastomeric polymer, silicone rubbers,fluoroelastomers, silicones such as Dow Corning HS3 as provided by DowCorning of Wilmington, Del., fluorosilicones such as Dow Corning 730 asprovided by Dow Corning of Wilmington, Del., etc, and acrylic polymerssuch as any acrylic in the 4900 VHB acrylic series as provided by 3MCorp. of St. Paul, Minn.

[0083] In many cases, materials used in accordance with the presentinvention are commercially available polymers. The commerciallyavailable polymers may include, for example, any commercially availablesilicone elastomer, polyurethane, PVDF copolymer and adhesive elastomer.Using commercially available materials provides cost-effectivealternatives for transducers and associated devices of the presentinvention. The use of commercially available materials may simplifyfabrication. In one embodiment, the commercially available polymer is acommercially available acrylic elastomer comprising mixtures ofaliphatic acrylate that are photocured during fabrication. Theelasticity of the acrylic elastomer results from a combination of thebranched aliphatic groups and cross-linking between the acrylic polymerchains.

[0084] Materials used as a pre-strained polymer may be selected based onone or more material properties such as a high electricalbreakdownstrength, a low modulus of elasticity—for large or smalldeformations, a high dielectric constant, etc. In one embodiment, thepolymer is selected such that is has an elastic modulus below 100 MPa.In another embodiment, the polymer is selected such that is has amaximum actuation pressure between about 0.05 MPa and about 10 MPa, andpreferably between about 0.3 MPa and about 3 MPa. In another embodiment,the polymer is selected such that is has a dielectric constant betweenabout 2 and about 20, and preferably between about 2.5 and about 12. Forsome applications, an electroactive polymer is selected based on one ormore application demands such as a wide temperature and/or humidityrange, repeatability, accuracy, low creep, reliability and endurance.

[0085] Suitable actuation voltages for pre-strained polymers of thepresent invention may vary based on the electroactive polymer materialand its properties (e.g. the dielectric constant) as well as thedimensions of the polymer (e.g. the thickness between electrodes). Byway of example, actuation electric fields for the polymer 102 in FIG. 1Amay range in magnitude from about 0 V/m to 440 MegaVolts/meter.Actuation voltages in this range may produce a pressure in the range ofabout 0 Pa to about 10 MPa. To achieve a transducer capable of higherforces, the thickness of the polymer may be increased. Alternatively,multiple polymer layers may be implemented. Actuation voltages for aparticular polymer may be reduced by increasing the dielectric constant,decreasing polymer thickness and decreasing the modulus of elasticity,for example.

[0086] Pre-strained polymers of the present invention may cover a widerange of thicknesses. In one embodiment, polymer thickness may rangebetween about 1 micrometerand 2 millimeters. Typical thicknesses beforepre-strain include 50-225 micrometers for HS3, 25-75 micrometers forNuSil CF 19-2186, and 100-1000 micrometers for any of the 3M VHB 4900series acrylic polymers. Polymer thickness may be reduced by stretchingthe film in one or both planar directions. In many cases, pre-strainedpolymers of the present invention may be fabricated and implemented asthin films. Thicknesses suitable for these thin films may be below 50micrometers.

[0087] 3. Actuators

[0088] The deflection of a pre-strained polymer can be used in a varietyof ways to produce mechanical energy. Generally speaking, electroactivepolymers of the present invention may be implemented with a variety ofactuators—including conventional actuators retrofitted with apre-strained polymer and custom actuators specially designed for one ormore pre-strained polymers. Conventional actuators include extenders,bending beams, stacks, diaphragms, etc. Several different exemplarycustom actuators in accordance with the present invention will now bediscussed.

[0089]FIG. 1E illustrates a cross-sectional side view of a diaphragmactuator 130 including a pre-strained polymer 131 before actuation inaccordance with one embodiment of the present invention. Thepre-strained polymer 131 is attached to a frame 132. The frame 132includes a circular hole 133 that allows deflection of the polymer 131perpendicular to the area of the circular hole 133. The diaphragmactuator 130 includes circular electrodes 134 and 136 on either side ofthe polymer 131 to provide a voltage difference across a portion of thepolymer 131.

[0090] In the voltage-off configuration of FIG. 1E, the polymer 131 isstretched and secured to the frame 132 with tension to achievepre-strain. Upon application of a suitable voltage to the electrodes 134and 136, the polymer film 131 expands away from the plane of the frame132 as illustrated in FIG. 1F. The electrodes 134 and 136 are compliantand change shape with the pre-strained polymer 131 as it deflects.

[0091] The diaphragm actuator 130 is capable of expansion in bothdirections away from the plane. In one embodiment, the bottom side 141of the polymer 131 includes a bias pressure that influences theexpansion of the polymer film 131 to continually actuate upward in thedirection of arrows 143 (FIG. 1F). In another embodiment, a swellingagent such as a small amount of silicone oil is applied to the bottomside 141 to influence the expansion of the polymer 131 in the directionof arrows 143. The swelling agent causes slight permanent deflection inone direction as determined during fabrication, e.g. by supplying aslight pressure on the bottom side 141 when the swelling agent isapplied. The swelling agent allows the diaphragm to continually actuatein a desired direction without using a bias pressure.

[0092] The amount of expansion for the diaphragm actuator 130 will varybased on a number of factors including the polymer 131 material, theapplied voltage, the amount of pre-strain, any bias pressure, complianceof the electrodes 134 and 136, etc. In one embodiment, the polymer 131is capable of deflections to a height 137 of at least about 50 percentof the hole diameter 139 and may take a hemispheric shape at largedeflections. In this case, an angle 147 formed between the polymer 131and the frame 132 may be less than 90 degrees.

[0093] As mentioned earlier, expansion in one direction of anelectroactive polymer may induce contractile stresses in a seconddirection such as due to Poisson effects. This may reduce the mechanicaloutput for a transducer that provides mechanical output in the seconddirection. Correspondingly, actuators of the present invention may bedesigned to constrain a polymer in the non-output direction. In somecases, actuators may be designed to improve mechanical output usingdeflection in the non-output direction.

[0094] An actuator which uses deflection in one planar direction toimprove mechanical output in the other planar direction is a bowactuator. FIGS. 2A and 2B illustrate a bow actuator 200 before and afteractuation in accordance with a specific embodiment of the presentinvention. The bow actuator 200 is a planar mechanism comprising aflexible frame 202 which provides mechanical assistance to improvemechanical output for a polymer 206 attached to the frame 202. The frame202 includes six rigid members 204 connected at joints 205. The members204 and joints 205 provide mechanical assistance by coupling polymerdeflection in a planar direction 208 into mechanical output in aperpendicular planar direction 210. More specifically, the frame 202 isarranged such that a small deflection of the polymer 206 in thedirection 208 improves displacement in the perpendicular planardirection 210. Attached to opposite (top and bottom) surfaces of thepolymer 206 are electrodes 207 (bottom electrode on bottom side ofpolymer 206 not shown) to provide a voltage difference across a portionof the polymer 206.

[0095] The polymer 206 is configured with different levels of pre-strainin its orthogonal directions. More specifically, the electroactivepolymer 206 includes a high pre-strain in the planar direction 208, andlittle or no pre-strain in the perpendicular planar direction 210. Thisanisotropic pre-strain is arranged relative to the geometry of the frame202. More specifically, upon actuation across electrodes 207 and 209,the polymer contracts in the high pre-strained direction 208. With therestricted motion of the frame 202 and the lever arm provided by themembers 204, this contraction helps drive deflection in theperpendicular planar direction 210. Thus, even for a short deflection ofthe polymer 206 in the high pre-strain direction 208, the frame 202 bowsoutward in the direction 210. In this manner, a small contraction in thehigh pre-strain direction 210 becomes a larger expansion in therelatively low pre-strain direction 208.

[0096] Using the anisotropic pre-strain and constraint provided by theframe 202, the bow actuator 200 allows contraction in one direction toenhance mechanical deflection and electrical to mechanical conversion inanother. In other words, a load 211 (FIG. 2B) attached to the bowactuator 200 is coupled to deflection of the polymer 206 in twodirections—direction 208 and 210. Thus, as a result of the differentialpre-strain of the polymer 206 and the geometry of the frame 202, the bowactuator 200 is able to provide a larger mechanical displacement than anelectroactive polymer alone for common electrical input.

[0097] The bow actuator 200 may be configured based on the polymer 206.By way of example, the geometry of the frame 202 and dimensions of thepolymer 206 may be adapted based on the polymer 206 material. In aspecific embodiment using HS3 silicone as the polymer 206, the polymer206 preferably has a ratio in directions 208 and 210 of 9:2 withpre-strains about 270 percent and −25 percent in the directions 208 and210 respectively. Using this arrangement, linear strains of at leastabout 100 percent in direction 210 are possible.

[0098] The pre-strain in the polymer 206 and constraint provided by theframe 202 may also allow the bow actuator 200 to utilize lower actuationvoltages for the pre-strained polymer 206 for a given deflection. As thebow actuator 200 has a lower effective modulus of elasticity in the lowpre-strained direction 210, the mechanical constraint provided by theframe 202 allows the bow actuator 200 to be actuated in the direction210 to a larger deflection with a lower voltage. In addition, the highpre-strain in the direction 208 increases the breakdown strength of thepolymer 206, permitting higher voltages and higher deflections for thebow actuator 200.

[0099] As mentioned earlier with respect FIG. 1A, when a polymer expandsas a result of electrostatic forces, it continues to expand untilmechanical forces balance the electrostatic pressure driving theexpansion. When the load 211 is attached to the bow actuator 200,mechanical effects provided by the load 211 will influence the forcebalance and deflection of the polymer 206. For example, if the load 211resists expansion of the bow actuator 200, then the polymer 206 may notexpand as much as if were there no load.

[0100] In one embodiment, the bow actuator 200 may include additionalcomponents to provide mechanical assistance and enhance mechanicaloutput. By way of example, springs 220 as shown in FIG. 2C may beattached to the bow actuator 200 to enhance deflection in the direction210. The springs load the bow actuator 200 such that the spring forceexerted by the springs opposes resistance provided by an external load.In some cases, the springs 220 provide increasing assistance for bowactuator 200 deflection. In one embodiment, spring elements may be builtinto the joints 205 instead of the external springs 220 to enhancedeflection of the bow actuator 200. In addition, pre-strain may beincreased to enhance deflection. The load may also be coupled to therigid members 204 on top and bottom of the frame 202 rather than on therigid members of the side of the frame 202 (as shown in FIG. 2B). Sincethe top and bottom rigid members 204 contract towards each other whenvoltage is applied as shown in FIG. 2B, the bow actuator 200 provides anexemplary device contracts in the plane upon application of a voltagerather than expands.

[0101] Although the bow actuator 200 of FIGS. 2A-2C illustrates aspecific example of a custom actuator including a flexible frame and anelectroactive polymer, any frame geometry or mechanical assistance toimprove displacement of an electroactive polymer is suitable for usewith the present invention.

[0102] The shape and constraint of the polymer may affect deflection. Anaspect ratio for an electroactive polymer is defined as the ratio of itslength to width. If the aspect ratio is high (e.g., an aspect ratio ofat least about 4:1) and the polymer is constrained along its length byrigid members, than the combination may result in a substantially onedimensional deflection in the width direction.

[0103]FIGS. 2D and 2E illustrate a linear motion actuator 230 before andafter actuation in accordance with a specific embodiment of the presentinvention. The linear motion actuator 230 is a planar mechanismproviding mechanical output in one direction. The linear motion actuator230 comprises a polymer 231 having a length 233 substantially greaterthan its width 234 (e.g., an aspect ratio of at least about 4:1). Thepolymer 231 is attached on opposite sides to stiff members 232 of aframe along its length 233. The stiff members 232 have a greaterstiffness than the polymer 231. The geometric edge constraint providedby the stiff members 232 substantially prevents displacement in adirection 236 along the polymer length 233 and facilitates deflectionalmost exclusively in a direction 235. When the linear motion actuator230 is implemented with a polymer 231 having anisotropic pre-strain,such as a higher pre-strain in the direction 236 than in the direction235, then the polymer 231 is stiffer in the direction 236 than in thedirection 235 and large deflections in the direction 235 may result. Byway of example, such an arrangement may produce linear strains of atleast about 200 percent for acrylics having an anisotropic pre-strain.

[0104] A collection of electroactive polymers or actuators may bemechanically linked to form a larger actuator with a common output, e.g.force and/or displacement. By using a small electroactive polymer as abase unit in a collection, conversion of electric energy to mechanicalenergy may be scaled according to an application. By way of example,multiple linear motion actuators 230 may be combined in series in thedirection 235 to form an actuator having a cumulative deflection of allthe linear motion actuators in the series. When transducing electricenergy into mechanical energy, electroactive polymers—eitherindividually or mechanically linked in a collection—may be referred toas ‘artificial muscle’. For purposes herein, artificial muscle isdefined as one or more transducers and/or actuators having a singleoutput force and/or displacement. Artificial muscle may be implementedon a micro or macro level and may comprise any one or more of theactuators described herein.

[0105]FIG. 2F illustrates cross-sectional side view of a multilayeractuator 240 as an example of artificial muscle in accordance with aspecific embodiment of the present invention. The multilayer actuator240 includes four pre-strained polymers 241 arranged in parallel andeach attached to a rigid frame 242 such that they have the samedeflection. Electrodes 243 and 244 are deposited on opposite surfaces ofeach polymer 241 and provide simultaneous electrostatic actuation to thefour pre-strained polymers 241. The multilayer actuator 240 providescumulative force output of the individual polymer layers 241.

[0106] In another embodiment, multiple electroactive polymer layers maybe used in place of one polymer to increase the force or pressure outputof an actuator. For example, ten electroactive polymers may be layeredto increase the pressure output of the diaphragm actuator of FIG. 1E.FIG. 2G illustrates such a stacked multilayer diaphragm actuator 245 asanother example of artificial muscle in accordance with one embodimentof the present invention. The stacked multilayer actuator 245 includesthree polymer layers 246 layered upon each other and may be attached byadhesive layers 247. Within the adhesive layers 247 are electrodes 248and 249 that provide actuation to polymer layers 246. A relatively rigidplate 250 is attached to the outermost polymer layer and patterned toinclude holes 251 that allow deflection for the stacked multilayerdiaphragm actuator 245. By combining the polymer layers 246, the stackedmultilayer actuator 245 provides cumulative force output of theindividual polymer layers 246.

[0107] In addition to the linear motion actuator 230 of FIGS. 2D and 2E,electroactive polymers of the present invention may be included in avariety of actuators that provide linear displacement. FIG. 2Hillustrates a linear actuator 255 comprising an electroactive polymerdiaphragm 256 in accordance with another embodiment of the presentinvention. In this case, an output shaft 257 is attached to a centralportion of the diaphragm 256 that deflects in a hole 258 of a frame 261.Upon actuation and removal of electrostatic energy, the output shaft 257translates as indicated by arrow 259. The linear actuator 255 may alsoinclude a compliant spring element 260 that helps position the outputshaft 257.

[0108] In another embodiment, pre-strained polymers of the presentinvention may be rolled or folded into linear transducers and actuatorsthat deflect axially upon actuation. As fabrication of electroactivepolymers is often simplest with fewer numbers of layers, rolledactuators provide an efficient manner of squeezing large layers ofpolymer into a compact shape. Rolled or folded transducers and actuatorsmay include one or more layers of polymer rolled or folded to providenumerous layers of polymer adjacent to each other. Rolled or foldedactuators are applicable wherever linear actuators are used, such asrobotic legs and fingers, high force grippers, and general-purposelinear actuators.

[0109]FIG. 2I illustrates an inchworm-type actuator 262 in accordancewith a specific embodiment of the present invention. The inchworm-typeactuator 262 includes two or more rolled pre-strained polymer layerswith electrodes 263 that deflect axially along its cylindrical axis. Theinchworm-type actuator 262 also includes electrostatic clamps 264 and265 for attaching and detaching to a metal surface 268. Theelectrostatic clamps 264 and 265 allow the total stroke for theinchworm-type actuator 262 to be increased compared to an actuatorwithout clamping. As the clamping force per unit weight for theelectrostatic clamps 264 and 265 is high, the force per unit weightadvantages of pre-strained polymers of the present invention arepreserved with the inchworm-type actuator 262. The electrostatic clamps264 and 265 are attached to the inchworm-type actuator at connectionregions 267. A body 266 of the inchworm-type actuator includes theconnection regions 267 and the polymer 263 and has a degree of freedomalong the axial direction of the rolled polymer 263 between theconnection regions 267. In one embodiment, the electrostatic clamps 264and 265 include an insulating adhesive 269 that prevents electricalshorting from the conductive electrostatic clamps 264 and 265 to themetal surface 268.

[0110] The inchworm-type actuator 262 moves upward in a six stepprocess. In step one, the inchworm-type actuator 262 is immobilized atits respective ends when both electrostatic clamps 264 and 265 areactuated and the polymer 263 is relaxed. An electrostatic clamp isactuated by applying a voltage difference between the clamp and themetal surface 268. In step two, clamp 265 is released. Releasing one ofthe clamps 264 and 265 allows its respective end of the inchworm-typeactuator 262 to move freely. In step three, the electroactive polymer263 is actuated and extends the inchworm-type actuator 262 upward. Instep four, clamp 265 is actuated and the inchworm-type actuator 262 isimmobilized. In step five, clamp 264 is released. In step six, thepolymer 263 is relaxed and the inchworm-type actuator 262 contracts. Bycyclically repeating steps one through six, the inchworm-type actuator262 moves in the upward direction. By switching clamps 264 and 265 inthe above six step process, the inchworm-type actuator 262 moves in areverse direction.

[0111] Although the inchworm-type actuator 262 has been described interms of actuation using a single electroactive polymer and two clamps,multiple segment inchworm-type actuators using multiple electroactivepolymers may be implemented. Multiple segment inchworm-type actuatorsallow an inchworm-type actuator to increase in length without becomingthicker. A two-segment inchworm-type actuator would use two rolledpolymers rather than one and three clamps rather than two. In general,an n-segment inchworm-type actuator comprises n actuators between n+1clamps.

[0112]FIG. 2J illustrates a stretched film actuator 270 for providinglinear deflection in accordance with another embodiment of the presentinvention. The stretched film actuator 270 includes a rigid frame 271having a hole 272. A pre-strained polymer 273 is attached in tension tothe frame 271 and spans the hole 272. A rigid bar 274 is attached to thecenter of the polymer 273 and provides external displacementcorresponding to deflection of the polymer 273. Compliant electrodepairs 275 and 276 are patterned on both top and bottom surfaces of thepolymer 273 on the left and right sides respectively of the rigid bar274. When the electrode pair 275 is actuated, a portion of the polymer273 between and in the vicinity of the top and bottom electrode pair 275expands relative to the rest of the polymer 273 and the existing tensionin the remainder of the polymer 273 pulls the rigid bar 274 to move tothe right. Conversely, when the electrode pair 276 is actuated, a secondportion of the polymer 273 affected by the electrode pair 276 expandsrelative to the rest of the polymer 273 and allows the rigid bar 274 tomove to the left. Alternating actuation of the electrodes 275 and 276provides an effectively larger total stroke 279 for the rigid bar 274.One variation of this actuator includes adding anisotropic pre-strain tothe polymer such that the polymer has high pre-strain (and stiffness) inthe direction perpendicular to the rigid bar displacement. Anothervariation is to eliminate one of the electrode pairs. For the benefit ofsimplifying the design, this variation reduces the stroke 279 for thestretched film actuator 270. In this case, the portion of the polymer nolonger used by the removed electrode now responds passively like arestoring spring.

[0113]FIG. 2K illustrates a bending beam actuator 280 in accordance withanother embodiment of the present invention. The bending beam actuator280 includes a polymer 281 fixed at one end by a rigid support 282 andattached to a flexible thin material 283 such as polyimide or mylarusing an adhesive layer, for example. The flexible thin material 283 hasa modulus of elasticity greater than the polymer 281. The difference inmodulus of elasticity for the top and bottom sides 286 and 287 of thebending beam actuator 280 causes the bending beam actuator 280 to bendupon actuation. Electrodes 284 and 285 are attached to the oppositesides of the polymer 281 to provide electrical energy. The bending beamactuator 280 includes a free end 288 having a single bending degree offreedom. Deflection of the free end 288 may be measured by thedifference in angle between the free end 288 and the end fixed by therigid support 282. FIG. 2L illustrates the bending beam actuator 280with a 90 degree bending angle.

[0114] The maximum bending angle for the bending beam actuator 280 willvary with a number of factors including the polymer material, theactuator length, the bending stiffness of the electrodes 284 and 285 andflexible thin material 283, etc. For a bending beam actuator 280comprising Dow Corning HS3 silicone, gold electrodes and an active areaof 3.5 mm in length, bending angles over 225 degrees are attainable. Forthe bending beam actuator 280, as the length of the active areaincreases, increased bending angles are attainable. Correspondingly, byextending the active length of the above mentioned bending beam actuatorto 5 mm allows for a bending angle approaching 360 degrees.

[0115] In one embodiment, one of the electrodes may act as the flexiblethin material 283. Any thin metal, such as gold, having a low bendingstiffness and a high tensile stiffness may be suitable for an electrodeacting as the flexible thin material 283. In another embodiment, abarrier layer is attached between one of the electrodes 284 and 285 andthe polymer 281 to minimize the effect of any localized breakdown in thepolymer. Breakdown may be defined as the point at which the polymercannot sustain the applied voltage. The barrier layer is typicallythinner than the polymer 281 and has a higher dielectric constant thanthe polymer 281 such that the voltage drop mainly occurs across thepolymer 281. It is often preferable that the barrier layer have a highdielectric breakdown strength.

[0116]FIG. 2M illustrates a bending beam actuator 290 in accordance withanother embodiment of the present invention. The bending beam actuator290 includes top and bottom pre-strained polymers 291 and 292 fixed atone end by a rigid support 296. Each of the polymers 291 and 292 may beindependently actuated. Independent actuation is achieved by separateelectrical control of top and bottom electrodes 293 and 294 attached tothe top and bottom electroactive polymers 291 and 292, respectively. Acommon electrode 295 is situated between the top and bottomelectroactive polymers 291 and 292 and attached to both. The commonelectrode 295 may be of sufficient stiffness to maintain the pre-strainon the polymer layers 291 and 292 while still permitting extension andbending.

[0117] Actuating the top electroactive polymer 291 using the top pair ofelectrodes 293 and 295 causes the bending beam actuator 290 to benddownward. Actuating the bottom polymer 292 using the bottom pair ofelectrodes 294 and 295 causes the bending beam actuator 290 to bendupward. Thus, independent use of the top and bottom electroactivepolymers 291 and 292 allows the bending beam actuator 290 to becontrolled along a radial direction 297. When both top and bottompolymers 291 and 292 are actuated simultaneously—and are ofsubstantially similar size and material—the bending beam actuator 290extends in length along the linear direction 298. Combining the abilityto control motion in the radial direction 297 and the linear direction298, the bending beam actuator 290 becomes a two-degree-of-freedomactuator. Correspondingly, independent actuation and control of the topand bottom polymers 291 and 292 allows a free end 299 of the bendingbeam actuator 290 to execute complex motions such as circular orelliptical paths.

[0118] 4. Performance

[0119] A transducer in accordance with the present invention convertsbetween electrical energy and mechanical energy. Transducer performancemay be characterized in terms of the transducer by itself, theperformance of the transducer in an actuator, or the performance of thetransducer in a specific application (e.g., a number of transducersimplemented in a motor). Pre-straining electroactive polymers inaccordance with the present invention provides substantial improvementsin transducer performance.

[0120] Characterizing the performance of a transducer by itself usuallyrelates to the material properties of the polymer and electrodes.Performance of an electroactive polymer may be described independent ofpolymer size by parameters such as strain, energy density, actuationpressure, actuation pressure density and efficiency. It should be notedthat the performance characterization of pre-strained polymers and theirrespective transducers described below may vary for differentelectroactive polymers and electrodes.

[0121] Pre-strained polymers of the present invention may have aneffective modulus in the range of about 0.1 to about 100 MPa. Actuationpressure is defined as the change in force within a pre-strained polymerper unit cross-sectional area between actuated and unactuated states. Insome cases, pre-strained polymers of the present invention may have anactuation pressure in the range of about 0 to about 100 MPa, and morepreferably in the range 0.1 to 10 MPa. Specific elastic energydensity—defined as the energy of deformation of a unit mass of thematerial in the transition between actuated and unactuated states—mayalso be used to describe an electroactive polymer where weight isimportant. Pre-strained polymers of the present invention may have aspecific elastic energy density of over 3 J/g.

[0122] The performance of a pre-strained polymer may also be described,independent of polymer size, by efficiency. Electromechanical efficiencyis defined as the ratio of mechanical output energy to electrical inputenergy. Electromechanical efficiency greater than 80 percent isachievable with some pre-strained polymers of the present invention. Thetime for a pre-strained polymer to rise (or fall) to its maximum (orminimum) actuation pressure is referred to as its response time.Pre-strained polymer polymers in accordance with the present inventionmay accommodate a wide range of response times. Depending on the sizeand configuration of the polymer, response times may range from about0.01 milliseconds to 1 second, for example. A pre-strained polymerexcited at a high rate may also be characterized by an operationalfrequency. In one embodiment, maximum operational frequencies suitablefor use with the present invention may be in the range of about 100 Hzto 100 kHz. Operational frequencies in this range allow pre-strainedpolymers of the present invention to be used in various acousticapplications (e.g., speakers). In some embodiments, pre-strainedpolymers of the present invention may be operated at a resonantfrequency to improve mechanical output.

[0123] Performance of an actuator may be described by a performanceparameter specific to the actuator. By way of example, performance of anactuator of a certain size and weight may be quantified by parameterssuch as stroke or displacement, force, actuator response time.Characterizing the performance of a transducer in an application relatesto how well the transducer is embodied in a particular application (e.g.in robotics). Performance of a transducer in an application may bedescribed by a performance parameter specific to the application (e.g.,force/unit weight in robotic applications). Application specificparameters include stroke or displacement, force, actuator responsetime, frequency response, efficiency, etc. These parameters may dependon the size, mass and/or the design of the transducer and the particularapplication.

[0124] It should be noted that desirable material properties for anelectroactive polymer may vary with an actuator or application. Toproduce a large actuation pressures and large strain for an application,a pre-strained polymer may be implemented with one of a high dielectricstrength, a high dielectric constant, and a low modulus of elasticity.Additionally, a polymer may include one of a high-volume resistivity andlow mechanical damping for maximizing energy efficiency for anapplication.

[0125] 5. Electrodes

[0126] As mentioned above, transducers of the present inventionpreferably include one or more electrodes for actuating an electroactivepolymer. Generally speaking, electrodes suitable for use with thepresent invention may be of any shape and material provided they areable to supply or receive a suitable voltage, either constant or varyingover time, to or from an electroactive polymer. In one embodiment, theelectrodes adhere to a surface of the polymer. Electrodes adhering tothe polymer are preferably compliant and conform to the changing shapeof the polymer. The electrodes may be only applied to a portion of anelectroactive polymer and define an active area according to theirgeometry.

[0127] The compliant electrodes are capable of deflection in one or moredirections. Linear strain may be used to describe the deflection of acompliant electrode in one of these directions. As the term is usedherein, linear strain of a compliant electrode refers to the deflectionper unit length along a line of deflection. Maximum linear strains(tensile or compressive) of at least about 50 percent are possible forcompliant electrodes of the present invention. For some compliantelectrodes, maximum linear strains of at least about 100 percent arecommon. Of course, an electrode may deflect with a strain less than themaximum. In one embodiment, the compliant electrode is a ‘structuredelectrode’ that comprises one or more regions of high conductivity andone or more regions of low conductivity.

[0128]FIG. 3 illustrates a top surface view of a structured electrode501 that provides one-directional compliance in accordance with oneembodiment of the present invention. The structured electrode 501includes metal traces 502 patterned in parallel lines over a chargedistribution layer 503—both of which cover an active area of a polymer(not shown). The metal traces 502 and charge distribution layer 503 areapplied to opposite surfaces of the polymer. Thus, the cross section,from top to bottom, of a transducer including structured electrodes 501on opposite surfaces is: top metal traces, top charge distributionlayer, polymer, bottom charge distribution layer, bottom metal traces.Metal traces 502 on either surface of the polymer act as electrodes forelectroactive polymer material between them. In another embodiment, thebottom electrode may be a compliant, uniform electrode. The chargedistribution layer 503 facilitates distribution of charge between metaltraces 502. Together, the high conductivity metal traces 502 quicklyconduct charge across the active area to the low conductivity chargedistribution layer 503 which distributes the charge uniformly across thesurface of the polymer between the traces 502. The charge distributionlayer 503 is compliant. As a result, the structured electrode 501 allowsdeflection in a compliant direction 506 perpendicular to the parallelmetal traces 502.

[0129] Actuation, for the entire polymer may be achieved by extendingthe length of the parallel metal traces 502 across the length of thepolymer and by implementing a suitable number of traces 502 across thepolymer width. In one embodiment, the metal traces 502 are spaced atintervals in the order of 400 micrometers and have a thickness of about20 to 100 nanometers. The width of the traces is typically much lessthan the spacing. To increase the overall speed of response for thestructured electrode 501, the distance between metal traces 502 may bereduced. The metal traces 502 may comprise gold, silver, aluminum andmany other metals and relatively rigid conductive materials. In oneembodiment, metal traces on opposite surfaces of an electroactivepolymer are offset from one another to improve charge distributionthrough the polymer layer and prevent direct metal-to-metal electricalbreakdowns.

[0130] Deflection of the parallel metal traces 502 along their axisgreater than the elastic allowance of the metal trace material may leadto damage of the metal traces 502. To prevent damage in this manner, thepolymer may be constrained by a rigid structure that prevents deflectionof the polymer and the metal traces 502 along their axis. The rigidmembers 232 of the linear motion actuator of FIGS. 2D and 2E aresuitable in this regard. In another embodiment, the metal traces 502 maybe undulated slightly on the surface of the polymer 500. Theseundulations add compliance to the traces 502 along their axis and allowdeflection in this direction.

[0131] In general, the charge distribution layer 503 has a conductancegreater than the electroactive polymer but less than the metal traces.The non-stringent conductivity requirements of the charge distributionlayer 503 allow a wide variety of materials to be used. By way ofexample, the charge distribution layer may comprise carbon black,fluoroelastomer with colloidal silver, a water-based latex rubberemulsion with a small percentage in mass loading of sodium iodide, andpolyurethane with tetrathiafulavalene/tetracyanoquinodimethane(TTF/TCNQ) charge transfer complex. These materials are able to formthin uniform layers with even coverage and have a surface conductivitysufficient to conduct the charge between metal traces 502 beforesubstantial charge leaks into the surroundings. In one embodiment,material for the charge distribution layer 503 is selected based on theRC time constant of the polymer. By way of example, surface resistivityfor the charge distribution layer 503 suitable for the present inventionmay be in the range of 10⁶-10¹¹ ohms. It should also be noted that insome embodiments, a charge distribution layer is not used and the metaltraces 502 are patterned directly on the polymer. In this case, air oranother chemical species on the polymer surface may be sufficient tocarry charge between the traces. This effect may be enhanced byincreasing the surface conductivity through surface treatments such asplasma etching or ion implantation.

[0132] In another embodiment, multiple metal electrodes are situated onthe same side of a polymer and extend the width of the polymer. Theelectrodes provide compliance in the direction perpendicular to width.Two adjacent metal electrodes act as electrodes for polymer materialbetween them. The multiple metal electrodes alternate in this manner andalternating electrodes may be in electrical communication to providesynchronous activation of the polymer.

[0133]FIG. 4 illustrates a pre-strained polymer 510 underlying astructured electrode that is not directionally compliant according to aspecific embodiment of the present invention. The structured electrodeincludes metal traces 512 patterned directly on one surface of theelectroactive polymer 510 in evenly spaced parallel lines forming a‘zig-zag’ pattern. Two metal traces 512 on opposite surfaces of thepolymer act as electrodes for the electroactive polymer 510 materialbetween them. The ‘zig-zag’ pattern of the metal traces 512 allows forexpansion and contraction of the polymer and the structure electrode inmultiple directions 514 and 516.

[0134] Using an array of metal traces as described with respect to FIGS.3 and 4 permits the use of charge distribution layers having a lowerconductance. More specifically, as the spacing between metal tracesdecreases, the required conductance of the material between the tracesmay diminish. In this manner, it is possible to use materials that arenot normally considered conductive to be used as a charge distributionlayers. By way of example, polymers having a surface resistivity of 10¹⁰ohms may be used as an charge distribution layer in this manner. In aspecific embodiment, rubber was used as a charge distribution layer aspart of a structured electrode on a polymer layer having a thickness of25 micrometers and spacing between parallel metal traces of about 500micrometers. In addition to reducing the required conductance for acharge distribution layer, closely spaced metal traces also increase thespeed of actuation since the charge need only travel through the chargedistribution layer for a short distance between closely spaced metaltraces.

[0135] Although structured electrodes of the present invention have beendescribed in terms of two specific metal trace configurations;structured electrodes in accordance with the present invention may bepatterned in any suitable manner. As one skilled in the art willappreciate, various uniformly distributed metallic trace patterns mayprovide charge on the surface of a polymer while providing compliance inone or more directions. In some cases, a structured electrode may beattached to the surface of polymer in a non-uniform manner. As actuationof the polymer may be limited to an active region within suitableproximity of a pair of patterned metal traces, specialized active andnon-active regions for an electroactive polymer may be defined by custompatterning of the metal traces. These active and non-active regions maybe formed to custom geometries and high resolutions according toconventional metal trace deposition techniques. Extending this practiceacross the entire surface of an electroactive polymer, custom patternsfor structured electrodes comprising numerous custom geometry activeregions may result in specialized and non-uniform actuation of theelectroactive polymer according to the pattern of the structuredelectrodes.

[0136] Although the present invention has been discussed primarily interms of flat electrodes, ‘textured electrodes’ comprising varying outof plane dimensions may be used to provide a compliant electrode. FIG. 5illustrates exemplary textured electrodes 520 and 521 in accordance withone embodiment of the present invention. The textured electrodes 520 and521 are attached to opposite surfaces of an electroactive polymer 522such that deflection of the polymer 522 results in planar and non-planardeformation of the textured electrodes 520 and 521. The planar andnon-planar compliance of the electrodes 520 and 521 is provided by anundulating pattern which, upon planar and/or thickness deflection of thepolymer 522, provides directional compliance in a direction 526. Toprovide substantially uniform compliance for the textured electrodes 520and 521, the undulating pattern is implemented across the entire surfaceof the electroactive polymer in the direction 526. In one embodiment,the textured electrodes 520 and 521 are comprised of metal having athickness which allows bending without cracking of the metal to providecompliance. Typically, the textured electrode 520 is configured suchthat non-planar deflection of the electrodes 520 and 521 is much lessthan the thickness of the polymer 522 in order to provide asubstantially constant electric field to the polymer 522. Texturedelectrodes may provide compliance in more than one direction. In aspecific embodiment, a rough textured electrode provides compliance inorthogonal planar directions. The rough textured electrode may have atopography similar to the rough surface of FIG. 1D.

[0137] In one embodiment, compliant electrodes of the present inventioncomprise a conductive grease such as carbon grease or silver grease. Theconductive grease provides compliance in multiple directions. Particlesmay be added to increase the conductivity of the polymer. By way ofexample, carbon particles may be combined with a polymer binder such assilicone to produce a carbon grease that has low elasticity and highconductivity. Other materials may be blended into the conductive greaseto alter one or more material properties. Conductive greases inaccordance with the present invention are suitable for deflections of atleast about 100 percent strain.

[0138] Compliant electrodes of the present invention may also includecolloidal suspensions. Colloidal suspensions contain submicrometer sizedparticles, such as graphite, silver and gold, in a liquid vehicle.Generally speaking, any colloidal suspension having sufficient loadingof conductive particles may be used as an electrode in accordance withthe present invention. In a specific embodiment, a conductive greaseincluding colloidal sized conductive particles is mixed with aconductive silicone including colloidal sized conductive particles in asilicone binder to produce a colloidal suspension that cures to form aconductive semi-solid. An advantage of colloidal suspensions is thatthey may be patterned on the surface of a polymer by spraying, dipcoating and other techniques that allow for a thin uniform coating of aliquid. To facilitate adhesion between the polymer and an electrode, abinder may be added to the electrode. By way of example, a water-basedlatex rubber or silicone may be added as a binder to a colloidalsuspension including graphite.

[0139] In another embodiment, compliant electrodes are achieved using ahigh aspect ratio conductive material such as carbon fibrils and carbonnanotubes. These high aspect ratio carbon materials may form highsurface conductivities in thin layers. High aspect ratio carbonmaterials may impart high conductivity to the surface of the polymer atrelatively low electrode thicknesses due to the high interconnectivityof the high aspect ratio carbon materials. By way of example,thicknesses for electrodes made with common forms of carbon that are nothigh-aspect ratio may be in the range of 5-50 micrometers whilethicknesses for electrodes made with carbon fibril or carbon nanotubeelectrodes may be less than 2-4 micrometers. Area expansions well over100 percent in multiple directions are suitable with carbon fibril andcarbon nanotube electrodes on acrylic and other polymers. High aspectratio carbon materials may include the use of a polymer binder toincrease adhesion with the electroactive polymer layer. Advantageously,the use of polymer binder allows a specific binder to be selected basedon adhesion with a particular electroactive polymer layer and based onelastic and mechanical properties of the polymer.

[0140] In one embodiment, high-aspect-ratio carbon electrodes may befabricated thin enough such that the opacity of the electrodes may bevaried according to polymer deflection. By way of example, theelectrodes may be made thin enough such that the electrode changes fromopaque to semitransparent upon planar expansion. This ability tomanipulate the opacity of the electrode may allow transducers of thepresent invention to be applied to a number of various opticalapplications as will be described below.

[0141] In another embodiment, mixtures of ionically conductive materialsmay be used for the compliant electrodes. This may include, for example,water based polymer materials such as glycerol or salt in gelatin,iodine-doped natural rubbers and water-based emulsions to which organicsalts such as potassium iodide are added. For hydrophobic electroactivepolymers that may not adhere well to a water based electrode, thesurface of the polymer may be pretreated by plasma etching or with afine powder such as graphite or carbon black to increase adherence.

[0142] Materials used for the electrodes of the present invention mayvary greatly. Suitable materials used in an electrode may includegraphite, carbon black, colloidal suspensions, thin metals includingsilver and gold, silver filled and carbon filled gels. In a specificembodiment, an electrode suitable for use with the present inventioncomprises 80 percent carbon grease and 20 percent carbon black in asilicone rubber binder such as Stockwell RTV60-CON as produced byStockwell Rubber Co. Inc. of Philadelphia, Pa. The carbon grease is ofthe type such as Circuit Works 7200 as provided by ChemTronics Inc. ofKennesaw, Ga. The conductive grease may also be mixed with an elastomer,such as silicon elastomer RTV 118 as produced by General Electric ofWaterford, N.Y., to provide a gel-like conductive grease.

[0143] It is understood that certain electrode materials may work wellwith particular polymers and may not work as well for others. By way ofexample, carbon fibrils work well with acrylic elastomer polymers whilenot as well with silicone polymers. For most transducers, desirableproperties for the compliant electrode may include any one of a lowmodulus of elasticity, low mechanical damping, a low surfaceresistivity, uniform resistivity, chemical and environmental stability,chemical compatibility with the electroactive polymer, good adherence tothe electroactive polymer, and an ability to form smooth surfaces. Insome cases, it may be desirable for the electrode material to besuitable for precise patterning during fabrication. By way of example,the compliant electrode may be spray coated onto the polymer. In thiscase, material properties which benefit spray coating would bedesirable. In some cases, a transducer of the present invention mayimplement two different types of electrodes. By way of example, adiaphragm actuator of the present invention may have a structuredelectrode attached to its top surface and a high aspect ratio carbonmaterial deposited on the bottom side.

[0144] Electronic drivers are connected to the electrodes. The voltageprovided to electroactive polymer will depend upon specifics of anapplication. In one embodiment, a transducer of the present invention isdriven electrically by modulating an applied voltage about a DC biasvoltage. Modulation about a bias voltage allows for improved sensitivityand linearity of the transducer to the applied voltage. By way ofexample, a transducer used in an audio application may be driven by asignal of up to 200 to 1000 volts peak to peak on top of a bias voltageranging from about 750 to 2000 volts DC.

[0145] 6. Applications

[0146] As the present invention includes transducers that may beimplemented in both the micro and macro scales, and with a wide varietyof actuator designs, the present invention finds use in a broad range ofapplications where electrical energy is converted into mechanicalenergy. Provided below are several exemplary applications for some ofthe actuators described above. Broadly speaking, the transducers andactuators of the present invention may find use in any applicationrequiring conversion from mechanical to electrical energy.

[0147] As mentioned before, electroactive polymers, either individuallyor mechanically linked in a collection, may be referred to as artificialmuscle. The term artificial muscle in itself implies that theseactuators are well-suited for application to biologically inspiredrobots or biomedical applications where the duplication of muscle,mammalian or other, is desired. By way of example, applications such asprosthetic limbs, exoskeletons, and artificial hearts may benefit frompre-strained polymers of the present invention. The size scalability ofelectroactive polymers and the ability to use any number of transducersor polymer actuators in a collection allow artificial muscle inaccordance with the present invention to be used in a range inapplications greater than their biological counterparts. As transducersand actuators of the present invention have a performance range faroutside their biological counterparts, the present invention is notlimited to artificial muscle having a performance corresponding to realmuscle, and may indeed include applications requiring performanceoutside that of real muscle.

[0148] In one example of artificial muscle, a collection of linearmotion actuators comprises two or more layers of pre-strained polymersandwiched together and attached to two rigid plates at opposite edgesof each polymer. Electrodes are sealed into the center between each ofthe polymer layers. All of the linear motion actuators in the collectionmay take advantage of geometric constraints provided by the rigid platesand anisotropic pre-strain to restrict deformation of the polymer in theactuated direction. An advantage of the layered construction is that asmany electroactive polymer layers as required may be stacked in parallelin order to produce the desired force. Further, the stroke of thislinear motion actuator configuration may be increased by adding similarlinear motion actuators in series.

[0149] In the micro domain, the pre-strained polymers may range inthickness from several micrometers to several millimeters and preferablyfrom several micrometers to hundreds of micrometers. Micro pre-strainedpolymers are well-suited for applications such as inkjets, actuatedvalves, micropumps, inchworm-type actuators, pointing mirrors, soundgenerators, microclamps, and micro robotic applications. Micro roboticapplications may include micro robot legs, grippers, pointer actuatorsfor CCD cameras, wire feeders for micro welding and repair, clampingactuators to hold rigid positions, and ultrasonic actuators to transmitdata over measured distances. In another application, a diaphragmactuator may be implemented in an array of similar electroactive polymerdiaphragms in a planar configuration on a single surface. By way ofexample, an array may include sixty-two diaphragms with the diameter of150 micrometers each arranged in a planar configuration. In oneembodiment, the array of diaphragm actuators may be formed on a siliconwafer. Diaphragm actuator arrays produced in this manner may include,for example, from 5 to 10,000 or more diaphragms each having a diameterin the range of 60 to 150 micrometers. The array may be placed upon gridplates having suitably spaced holes for each diaphragm.

[0150] In the macro domain, each of the actuators described above may bewell suited to its own set of applications. For example, theinchworm-type actuator of FIG. 2I is suitable for use with small robotscapable of navigating through pipes less than 2 cm in diameter. Otheractuators are well-suited, for example, with applications such asrobotics, solenoids, sound generators, linear actuators, aerospaceactuators, and general automation.

[0151] In another embodiment, a transducer of the present invention isused as an optical modulation device or an optical switch. Thetransducer includes an electrode whose opacity varies with deflection. Atransparent or substantially translucent pre-strained polymer isattached to the opacity varying electrode and deflection of the polymeris used to modulate opacity of device. In the case of an optical switch,the opacity varying transducer interrupts a light source communicatingwith a light sensor. Thus, deflection of the transparent polymer causesthe opacity varying electrode to deflect and affect the light sensor. Ina specific embodiment, the opacity varying electrode includes carbonfibrils or carbon nanotubes that become less opaque as electrode areaincreases and the area fibril density decreases. In another specificembodiment, an optical modulation device comprised of an electroactivepolymer and an opacity varying electrode may be designed to preciselymodulate the amount of light transmitted through the device.

[0152] Diaphragm actuators may be used as pumps, valves, etc. In oneembodiment, a diaphragm actuator having a pre-strained polymer issuitable for use as a pump. Pumping action is created by repeatedlyactuating the polymer. Electroactive polymer pumps in accordance withthe present invention may be implemented both in micro and macro scales.By way of example, the diaphragm may be used as a pump having a diameterin the range of about 150 micrometers to about 2 centimeters. Thesepumps may include polymer strains over 100 percent and may producepressures of 20 kPa or more.

[0153]FIG. 6 illustrates a two-stage cascaded pumping system includingdiaphragm pumps 540 and 542 in accordance with a specific embodiment ofthe present invention. The diaphragm pumps 540 and 542 includepre-strained polymers 544 and 546 attached to frames 545 and 547. Thepolymers 544 and 546 deflect within holes 548 and 550 in the frames 545and 547 respectively in a direction perpendicular to the plane of theholes 548 and 550. The frames 545 and 547 along with the polymers 544and 546 define cavities 551 and 552. The pump 540 includes a plunger 553having a spring 560 for providing a bias to the diaphragm 544 towardsthe cavity 551.

[0154] A one-way valve 555 permits inlet of a fluid or gas into thecavity 551. A one-way valve 556 permits outlet of the fluid or gas outof the cavity 551 into the cavity 552. In addition, a one-way valve 558permits exit of the fluid or gas from the cavity 552. Upon actuation ofthe polymers 544 and 546, the polymers deflect in turn to change thepressure within the cavities 551 and 552 respectively, thereby movingfluid or gas from the one-way valve 555 to the cavity 551, out the valve556, into the cavity 552, and out the valve 558.

[0155] In the cascaded two-stage pumping system of FIG. 6, the diaphragmpump 542 does not include a bias since the pressurized output from thediaphragm pump 540 biases the pump 542. In one embodiment, only thefirst pump in a cascaded series of diaphragm pumps uses a biaspressure—or any other mechanism for self priming. In some embodiments,diaphragm pumps provided in an array may include voltages provided byelectronic timing to increase pumping efficiency. In the embodimentshown in FIG. 6, polymers 544 and 546 are actuated simultaneously forbest performance. For other embodiments which may involve more diaphragmpumps in the cascade, the electronic timing for the different actuatorsis ideally set so that one pump contracts in cavity volume while thenext pump in the series (as determined by the one-way valves) expands.In a specific embodiment, the diaphragm pump 540 supplies gas at a rateof 40 ml/min and a pressure about 1 kPa while the diaphragm pump 542supplies gas at substantially the same flow rate but increases thepressure to 2.5 kPa.

[0156] Bending beam actuators, such as those described with respect toFIGS. 2K-2M, may be used in a variety of commercial and aerospacedevices and applications such as fans, electrical switches and relays,and light scanners—on the micro and macro level. For bending beamactuators used as light scanners, a reflective surface such asaluminized mylar may be bonded to the free end of a bending beamactuator. More specifically, light is reflected when the bending beam isactuated and light passes when the bending beam is at rest. Thereflector may then be used to reflect incoming light and form a scannedbeam to form an arc or line according to the deflection of the actuator.Arrays of bending beam actuators may also be used for flat-paneldisplays, to control airflow over a surface, for low profile speakersand vibration suppressors, as “smart furs” for controlling heat transferand/or light absorption on a surface, and may act as cilia in acoordinated manner to manipulate objects.

[0157] Polymers and polymer films that are rolled into a tubular ormultilayer cylinder actuator may be implemented as a piston that expandsaxially upon actuation. Such an actuator is analogous to a hydraulic orpneumatic piston, and may be implemented in any device or applicationthat uses these traditional forms of linear deflection.

[0158] An electroactive polymer actuator may also operate at high speedsfor a variety of applications including sound generators and acousticspeakers, inkjet printers, fast MEMS switches etc. In a specificembodiment, an electroactive polymer diaphragm is used as a lightscanner. More specifically, a mirror may be placed on a flexure thatpushes down on a 5 mm diameter electroactive polymer diaphragm toprovide a mirrored flexure. Good scanning of images at a scanning anglefrom about 10 to 30 degrees may be accomplished with voltages in therange of about 190 to 300 volts and frequencies in the range of about 30to 300 Hz. Much larger scanning angles, up to 90 degrees for example,may also be accommodated using voltages in the range of 400 to 500 V. Inaddition, higher frequencies may be used with a stiffer mirroredflexure.

[0159] 7. Fabrication

[0160] As the pre-strained polymers may be implemented both in the microand macro scales, in a wide variety of actuator designs, with a widerange of materials, and in a broad range of applications, fabricationprocesses used with the present invention may vary greatly. In oneaspect, the present invention provides methods for fabricatingelectromechanical devices including one or more pre-strained polymers.

[0161]FIG. 7A illustrates a process flow 600 for fabricating anelectromechanical device having at least one electroactive polymer layerin accordance with one embodiment of the present invention. Processes inaccordance with the present invention may include up to severaladditional steps not described or illustrated here in order not toobscure the present invention. In some cases, fabrication processes ofthe present invention may include conventional materials and techniquessuch as commercially available polymers and techniques used infabrication of microelectronics and electronics technologies. Forexample, micro diaphragm actuators may be produced in situ on siliconusing conventional techniques to form the holes and apply the polymerand electrodes.

[0162] The process flow 600 begins by receiving or fabricating a polymer(602). The polymer may be received or fabricated according to severalmethods. In one embodiment, the polymer is a commercially availableproduct such as a commercially available acrylic elastomer film. Inanother embodiment, the polymer is a film produced by one of casting,dipping, spin coating or spraying. In one embodiment, the polymer isproduced while minimizing variations in thickness or any other defectsthat may compromise the maximize electric field that can be appliedacross the polymer and thus compromise performance.

[0163] Spin coating typically involves applying a polymer mixture on arigid substrate and spinning to a desired thickness. The polymer mixturemay include the polymer, a curing agent and a volatile dispersant orsolvent. The amount of dispersant, the volatility of the dispersant, andthe spin speed may be altered to produce a desired polymer. By way ofexample, polyurethane films may be spin coated in a solution ofpolyurethane and tetrahydrofuran (THF) or cyclohexanone. In the case ofsilicon substrates, the polymer may be spin coated on an aluminizedplastic or a silicon carbide. The aluminum and silicon carbide form asacrificial layer that is subsequently removed by a suitable etchant.Films in the range of one micrometer thick may been produced by spincoating in this manner. Spin coating of polymer films, such as silicone,may be done on a smooth non-sticking plastic substrate, such aspolymethyl methacrylate or teflon. The polymer film may then be releasedby mechanically peeling or with the assistance of alcohol or othersuitable release agent. Spin coating is also suitable for producingthicker polymers in the range of 10-750 micrometers. In some cases, thepolymer mixture may be centrifuged prior to spin coating to removeunwanted materials such as fillers, particulates, impurities andpigments used in commercial polymers. To increase centrifuge efficacy orto improve thickness consistency, a polymer may be diluted in a solventto lower its viscosity; e.g. silicone may be disbursed in naptha.

[0164] The polymer may then be pre-strained in one or more directions(604). In one embodiment, pre-strain is achieved by mechanicallystretching a polymer in or more directions and fixing it to one or moresolid members (e.g, rigid plates) while strained. Another technique formaintaining pre-strain includes the use of one or more stiffeners. Thestiffeners are long rigid structures placed on a polymer while it is ina pre-strained state, e.g. while it is stretched. The stiffenersmaintain the pre-strain along their axis. The stiffeners may be arrangedin parallel or other configurations to achieve directional compliance ofthe transducer. It should be noted that the increased stiffness alongthe stiffener axis comprises the increased stiffness provided by thestiffener material as well as the increased stiffness of the polymer inthe pre-strain direction.

[0165] Surfaces on the pre-strained polymer may be textured. In oneembodiment to provide texturing, a polymer is stretched more than it canstretch when actuated, and a thin layer of stiff material is depositedon the stretched polymer surface. For example, the stiff material may bea polymer that is cured while the electroactive polymer is stretched.After curing, the electroactive polymer is relaxed and the structurebuckles to provide the textured surface. The thickness of the stiffmaterial may be altered to provide texturing on any scale, includingsubmicrometer levels. In another embodiment, textured surfaces areproduced by reactive ion etching (RIE). By way of example, RIE may beperformed on a pre-strained polymer comprising silicon with an RIE gascomprising 90 percent carbon tetrafluoride and 10 percent oxygen to forma surface with wave troughs and crests 4 to 5 micrometers in depth.

[0166] One or more electrodes are then formed on the polymer (606). Forthe silicone polymer altered by RIE mentioned above, a thin layer ofgold may be sputter deposited on the RIE textured surface to provide atextured electrode. In another embodiment, one or more graphiteelectrodes may be patterned and deposited using a stencil. Electrodescomprising conductive greases mixed with a conductive silicone may befabricated by dissolving the conductive grease and the uncuredconductive silicone in a solvent. The solution may then be sprayed onthe electroactive polymer material and may include a mask or stencil toachieve a particular pattern.

[0167] The metal traces of the structured electrodes of FIGS. 3 and 4may be patterned photolithographically on top of the polymer or chargedistribution layer. By way of example, a layer of gold is sputterdeposited before depositing a photoresist over the gold. The photoresistand gold may be patterned according to conventional photolithographictechniques, e.g. using a mask followed by one or more rinses to removethe photoresist. A charge distribution layer added between the polymerand the metal traces may be deposited by spin coating, for example.

[0168] In a specific embodiment, a structured electrode is formed on apolymer by sputter depositing gold for about 2 to 3 minutes (accordingto a desired thickness) at about 150 angstroms per minute. HPR 506photoresist as provided by Arch Chemicals, of Norwalk, Conn. is thenspin coated on the gold at about 500 to 1500 rpm for about 20 to 30seconds and then baked at about 90 degrees Celsius. A mask is thenapplied before exposing the photoresist to UV light and development toremove unmasked portions of the photoresist. The gold is then etchedaway and the film is rinsed. The remaining photoresist is removed byexposure to UV light, development and rinsing. The gold traces may thenbe stretched to enhance strain tolerance.

[0169] Textured electrodes of the present invention may also bepatterned photolithographically. In this case, a photoresist isdeposited on a pre-strained polymer and patterned using a mask. Plasmaetching may remove portions of the electroactive polymer not protectedby the mask in a desired pattern. The mask may be subsequently removedby a suitable wet etch. The active surfaces of the polymer may then becovered with the thin layer of gold deposited by sputtering, forexample.

[0170] The transducer, comprising the one or more polymer layers andelectrodes, is then packaged according to an application (608).Packaging may also include assembly of multiple transducers mechanicallylinked or stacked as multiple layers. In addition, mechanical andelectrical connections to the transducers may be formed according to anapplication.

[0171] The present invention also provides alternative methods forfabricating electromechanical devices including multiple layers ofpre-strained polymer. In one embodiment, a process for fabricatingelectromechanical devices begins by obtaining or fabricating a polymerlayer. The polymer is then stretched to the desired pre-strain andattached to a first rigid frame. Next electrodes are deposited onto bothsides of the polymer so as to define active areas and establishelectrical connections. The electrodes may be patterned by a variety ofwell-known techniques such as spray coating through a mask. If desired,a second polymer layer is then stretched on a second frame. Electrodesare then patterned on this second polymer layer. The second polymerlayer is then coupled to the first layer by stacking their respectiveframes. Layers of suitable compliant adhesives may be used to bond thetwo layers and electrodes, if needed. The size of the frames is chosenso as not to interfere with the polymer layers making intimate contact.If interference is present, then it may be desirable to remove thesecond frame, e.g., by cutting away the polymer layer around theperiphery of the first frame. If desired, a third layer of polymer withelectrodes may be added in a manner similar to how the second layer wasadded to the first. This procedure may be continued until a desirednumber of layers is reached.

[0172] Rigid frames, rigid members or other electrical and mechanicalconnectors are then attached to the polymer layers, e.g., by gluing. Ifdesired, the polymer may then be removed from the first frame. In somecases, the first frame may serve as a structural part of the finalactuator or actuators. For example, the first frame may be an array ofholes to produce an array of diaphragm actuators.

[0173] FIGS. 7B-F illustrate a second process for fabricating anelectromechanical device 640 having multiple layers of electroactivepolymer in accordance with another embodiment of the present invention.Processes in accordance with the present invention may include up toseveral additional steps not described or illustrated here in order notto obscure the present invention. The process begins by producing apre-strained polymer 622 on a suitable rigid substrate 624, e.g. by spincoating a polymer on a polymethyl methacrylate (PMMA) disk, stretchingthe polymer (FIG. 7B) and then attaching it to rigid substrate 624.After the polymer 622 is cured, electrodes 625 are patterned on theexposed side 626 of the polymer 622. A solid member 627 such as aflexible film including one of polyimide, mylar or acetate film is thendeposited onto the electroactive polymer 622 (FIG. 7C) with a suitableadhesive 628.

[0174] The rigid substrate 624 is then released from the electroactivepolymer 622 (FIG. 7D). A releasing agent such as isopropyl alcohol maybe used to facilitate the release. Electrodes 629 are then patterned onthe previously unexposed side of the polymer 622. The assembly is thenbonded to another electroactive polymer layer 630 attached to a rigidsubstrate 631 (FIG. 7E). Polymer layers 622 and 630 may be bonded by anadhesive layer 632 comprising GE RTV 118 silicone, for example. Therigid substrate 631 is then released from the polymer 630 and electrodes633 are patterned on the available side 634 of the polymer 630. Ifadditional polymer layers are desired, the steps of adding a polymerlayer, removing the rigid substrate, and adding electrodes may berepeated to produce as many polymer layers as desired. Polymer layer 635has been added in this manner. To facilitate electrical communication toelectrodes in the inner layers of the device 640, a metal pin may bepushed through the structure to make contact with electrodes in eachlayer.

[0175] The solid member 627 may then be patterned or removed as neededto provide the frame or mechanical connections required by the specificactuator type. In one embodiment, diaphragm actuators may be formed bypatterning solid member 627 to form holes 636 which provide activeregions for the electromechanical device 640 using a suitable mask oretch technique (FIG. 7F). In another embodiment, if the active area isnot large and electrodes may be added to the active regions of thepolymers without damage, the solid member 627 may be patterned with theholes 636 prior to attachment to the polymer 622.

[0176] For the process of FIGS. 7B-F, the rigid substrate 624 istypically released from the electroactive polymer 622 by peeling theflexible electroactive polymer. Peeling is well-suited for fabricatingdevices comprising electroactive polymers with a substantially flatprofile. In another embodiment, sacrificial layers may be used betweenthe polymer or electrodes and the rigid substrate to facilitate release.The sacrificial layers allow the polymer, electrodes and attachedassembly to be released from a rigid substrate by etching away thesacrificial layer. Metals comprising aluminum and silver are suitablefor use as the sacrificial layers, for example. The use of metals allowsthe sacrificial layers to be etched away by liquids that do not affectthe polymer layers. Metal sacrificial layers may also be easilypatterned with various masking techniques to provide frames, connectorsfor other structural components for the electromechanical device 640.The sacrificial layers may also be used to fabricate devices comprisingtransducers with non flat profiles, e.g. using rigid substrates shapedas tubes. For geometrically complex transducers, sacrificial layers maybe used in combination with dip coating to provide the complex geometry.

[0177] Although fabrication of pre-strained polymers has been brieflydescribed with respect to a few specific examples, fabrication processesand techniques of the present invention may vary accordingly for any theactuators or applications described above. For example, the process forfabricating a diaphragm actuator in accordance with a specificembodiment may include spin coating a polymer on a substrate before astructured electrode is fabricated on the polymer. The polymer is thenstretched and rigid frames including one or more holes sized for theactive area of each diaphragm actuator are bonded to the pre-strainedpolymer, including any overlap portions of the structured electrode. Inanother embodiment, holes are etched into the substrate instead of usinga separate rigid frame, e.g. when the substrate is comprised of silicon.The substrate is then released from the polymer and an electrode isattached to the bottom side of the polymer.

[0178] 8. Conclusion

[0179] While this invention has been described in terms of severalpreferred embodiments, there are alterations, permutations, andequivalents that fall within the scope of this invention which have beenomitted for brevity's sake. By way of example, although the presentinvention has been described in terms of several numerous appliedmaterial electrodes, the present invention is not limited to thesematerials and in some cases may include air as an electrode. It istherefore intended that the scope of the invention should be determinedwith reference to the appended claims.

What is claimed is:
 1. An actuator for converting electrical energy intodisplacement in a first direction, the actuator comprising: at least onetransducer, each transducer comprising: at least two electrodes, and apolymer arranged in a manner which causes a portion of the polymer todeflect in response to a change in electric field; and one or more of 1)a flexible frame coupled to the polymer, the frame providing mechanicalassistance to improve displacement in the first direction, 2) at leastone stiff member coupled to the at least one transducer, the at leastone stiff member substantially preventing displacement in a seconddirection, 3) a frame coupled to the polymer wherein the frame and thepolymer are arranged in a manner which causes a location on the polymerto deflect within a plane in response to the change in the electricfield, 4) an output member coupled to the polymer and 5) combinationsthereof.
 2. The actuator of claim 1 wherein the mechanical assistancecomprises a set of springs.
 3. The actuator of claim 1 wherein themechanical assistance changes the resting position of the actuator. 4.The actuator of claim 1 wherein the polymer comprises pre-strain.
 5. Theactuator of claim 4 wherein the polymer comprises pre-strain in a seconddirection which improves displacement in the first direction.
 6. Theactuator of claim 5 wherein the actuator contracts in the seconddirection in response to the electric field.
 7. The actuator of claim 6wherein the flexible frame couples polymer deflection in the seconddirection into displacement in the first direction.
 8. The actuator ofclaim 1 wherein the polymer has a compliance in one direction greaterthan in a second.
 9. The actuator of claim 1 wherein the polymer has anaspect ratio of at least 4:1.
 10. The actuator of claim 1 wherein thepolymer has a dielectric constant between about 2 and about
 20. 11. Theactuator of claim 1 wherein the polymer comprises one of a siliconerubber and an acrylic.
 12. The actuator of claim 1 wherein the polymerhas a thickness between about 1 micrometer and 2 millimeters.
 13. Theactuator of claim 1 wherein the polymer has an elastic modulus belowabout 100 MPa.
 14. The actuator of claim 1 including the frame coupledto the polymer and further comprising a second electrode pair coupled toa second portion of the polymer arranged in a manner which causes thesecond portion of the polymer to deflect in response to the change inthe electric field and an output member coupled to the location whereinthe location is between the portion and the second portion of thepolymer.
 15. The actuator of claim 1 wherein the transducer is includedin an artificial muscle.
 16. The actuator of claim 1, furthercomprising: a support structure for securing the portion of the polymerat a first position wherein the portion of the polymer is stretched froman initial surface area to a first surface area to improve themechanical response of the transducer when it deflects from the firstposition to a second position and wherein the support structure is forsupplying a force to the stretched portion of the polymer that preventsthe stretched portion of the polymer from returning from the firstsurface area to about its initial surface area.
 17. The actuator ofclaim 16 wherein a ratio of the first surface area to the initialsurface area is in the range of about 1.5 to 50
 18. An actuator forconverting electrical energy into mechanical energy, the actuatorcomprising: a transducer comprising: at least two electrodes and apolymer arranged in a manner which causes a first portion of the polymerto deflect in response to a change in electric field provided by the atleast two electrodes; and one or more of 1) a flexible member havingfixed end and a free end coupled to the actuator, 2) a frame attached toa second portion of the polymer, the frame comprising at least onecircular hole, wherein the first portion deflects out of the plane ofthe at least one circular hole in response to the change in electricfield, 3) a body having at least one degree of freedom between a firstbody portion and a second body portion, the body including thetransducer attached to the first body portion and the second bodyportion and a first clamp attached to the first body portion and asecond clamp attached to the second body portion and 4) combinationsthereof.
 19. The actuator of claim 18 wherein the flexible member is asecond transducer.
 20. The actuator of claim 18 wherein the free end hastwo degrees of freedom.
 21. The actuator of claim 18 wherein theflexible member has a stiffness greater than the polymer.
 22. Theactuator of claim 18 further comprising a bias pressure added to a firstside of the polymer.
 23. The actuator of claim 22 wherein the biaspressure is provided by a swelling agent.
 24. The actuator of claim 18wherein the actuator is included in a pump.
 25. The actuator of claim 18wherein the first portion deflects at least partially through the hole.26. The actuator of claim 25 wherein the first portion deflects at leastpartially through the hole to a height greater than half the holediameter.
 27. The actuator of claim 18 wherein the actuator is used toprovide linear output.
 28. The actuator of claim 18 wherein the actuatoris included in an array of actuators.
 29. The actuator of claim 18wherein the polymer is rolled.
 30. The actuator of claim 18 wherein thefirst and second clamps are electrostatic clamps.
 31. The actuator ofclaim 18 wherein the polymer has a dielectric constant between about 2and about
 20. 32. The actuator of claim 18 wherein the polymer comprisesone of a silicone rubber and an acrylic.
 33. The actuator of claim 18wherein the polymer has a thickness between about 1 micrometer and 2millimeters.
 34. The actuator of claim 18 wherein the polymer has anelastic modulus below about 100 MPa.
 35. The actuator of claim 18wherein a portion of the polymer deflects out of the plane of thepolymer in response to the change in electric field.
 36. The actuator ofclaim 18 wherein the transducer is included in an artificial muscle. 37.The actuator of claim 18, further comprising: a support structure forsecuring the portion of the polymer at a first position wherein theportion of the polymer is stretched from an initial surface area to afirst surface area to improve the mechanical response of the transducerwhen it deflects from the first position to a second position andwherein the support structure is for supplying a force to the stretchedportion of the polymer that prevents the stretched portion of thepolymer from returning from the first surface area to about its initialsurface area.
 38. The actuator of claim 37 wherein a ratio of the firstsurface area to the initial surface area is in the range of about 1.5 to50.
 39. The actuator of claim 18 wherein the polymer comprisespre-strain.
 40. The actuator of claim 39 wherein the polymer comprisespre-strain in a second direction which improves displacement in thefirst direction.
 41. A transducer for converting from electrical energyto mechanical energy, the transducer comprising: at least twoelectrodes; a polymer arranged in a manner which causes a portion of thepolymer to deflect from a first position with a first surface area to asecond position with a second surface area in response to a change inelectric field; a support structure for securing the portion of thepolymer at the first position wherein the portion of the polymer isstretched from an initial surface area to the first surface area toimprove the mechanical response of the transducer when it deflects fromthe first position to the second position and wherein the supportstructure is for supplying a force to the stretched portion of thepolymer that prevents the stretched portion of the polymer fromreturning from the first surface area to about its initial surface areaand wherein a ratio of the first surface area to the initial surfacearea is in the range of about 1.5 to
 50. 42. The transducer of claim 41further comprising a barrier layer.
 43. The transducer of claim 41further comprising a stiff member attached to a portion of the polymer.44. The transducer of claim 43 wherein the stiff member is included in aframe.
 45. The transducer of claim 41 further comprising a secondpolymer arranged in a manner which causes a portion of the secondpolymer to deflect in response to a second change in electric field andthe second polymer is coupled to the polymer.
 46. The transducer ofclaim 45 wherein the second polymer is mechanically coupled to the firstpolymer such that they have the same deflection.